1. Field of the Invention
The present invention relates to a piezoelectric ultrasonic motor that includes a metal body and a plurality of piezoelectric plates attached to the metal body and converts simple vibrations of the piezoelectric plates into linear movement, and more particularly to a piezoelectric ultrasonic motor drive circuit that performs four-phase driving of the motor through self-oscillation, achieving a high efficiency, high output operation at any time, regardless of environmental factors.
2. Description of the Related Art
Piezoelectric ultrasonic motors convert simple vibration of piezoelectric ceramic elements, which contract and expand when current is applied thereto, into linear movement so as to serve as rotary motors. Piezoelectric ultrasonic motors are small and light weight. Because they use ultrasonic power, piezoelectric ultrasonic motors cause no noise and little electromagnetic interference. Due to these advantages, piezoelectric ultrasonic motors are typically used in lens drive units of expensive cameras.
Various types of piezoelectric ultrasonic motors have been proposed. FIG. 1 shows an example piezoelectric ultrasonic motor. As shown in FIG. 1, the piezoelectric ultrasonic motor basically comprises a metal body 1 serving as a stator and a plurality of piezoelectric plates 2 to 5 attached to surfaces of the metal body 1. The piezoelectric palates 2 to 5 contract and expand if voltage is applied to the piezoelectric palates 2 to 5 through electrodes 2a to 5a. If a voltage of the same frequency as the electromechanical resonant frequency of each of the piezoelectric plates 2 to 5 is applied to each of the piezoelectric plates 2 to 5, the amplitude of contraction and expansion movement of each of the piezoelectric plates 2 to 5 is maximized.
The plurality of piezoelectric plates 2 to 5 having these characteristics are longitudinally attached to the metal body 1. The number of the piezoelectric plates 2 to 5 varies depending on the shape of the metal body 1 and the used driving scheme. If voltage signals out of phase with each other by certain degrees are applied to the piezoelectric plates 2 to 5, the piezoelectric plates 2 to 5 contract and expand to cause a bending deformation of the metal body 1, thereby rotating the central axis of the metal body 1.
Thus, the piezoelectric ultrasonic motor needs a drive circuit for applying suitable drive signals to the plurality of piezoelectric plates 2 to 5.
FIG. 2 shows a drive circuit of a rod-shaped piezoelectric ultrasonic motor according to a two-phase driving method proposed in U.S. Pat. No. 2,439,499. A transformer 6 transforms a 60 Hz power supply voltage and applies the transformed voltage to piezoelectric plates 2 and 3. A phase shift circuit composed of an inductor 7 and a capacitor 8 shifts the phase of an output voltage of the transformer 6 by 90 degrees and then applies the phase-shifted voltage to piezoelectric plates 4 and 5.
In the drive circuit of FIG. 2, the frequency of drive signal applied to the piezoelectric plates 2 to 5 may vary according to changes in the power supply voltage received from the outside. As described above, the piezoelectric ultrasonic motor can obtain the maximum efficiency and output only when receiving a drive voltage having the same frequency as the electromechanical resonant frequency of the piezoelectric plates 2 and 5. However, the conventional drive circuit of FIG. 2 has a problem in that mass productivity and driving efficiency are reduced if external environmental factors or frequency differences between products cause changes in the resonant frequency.
To overcome this problem, studies have been done to implement a self-oscillation function in the drive circuit so as to apply drive signals of the same frequency as the electromechanical resonant frequency, regardless of environmental factors and frequency differences between products. FIG. 3 shows an improved drive circuit having such a function.
As shown in FIG. 3, the conventional improved drive circuit comprises a feedback resistor Rf and an inverter INV, which are connected in parallel to a piezoelectric ultrasonic motor 30, and capacitors CL1 and CL2 which are connected between the ground and the piezoelectric ultrasonic motor 30. RC oscillation is performed according to capacitances of the capacitors CL1 and CL2, a capacitance caused by the piezoelectric ultrasonic motor 30, and a resistance of the feedback resistor Rf. That is, the oscillation frequency is determined based on the capacitances of the capacitors CL1 and CL2, the capacitance of the piezoelectric ultrasonic motor 30, and the resistance of the feedback resistor Rf. By suitably controlling the capacitances of the capacitors CL1 and CL2 and the resistance of the feedback resistor Rf, it is possible to make the oscillation frequency equal to the electromechanical resonant frequency of the piezoelectric ultrasonic motor 30.
If the oscillation frequency is set equal to the electromechanical resonant frequency of the piezoelectric ultrasonic motor 30 in such a manner, drive signals applied to the piezoelectric motor 30 always have the same frequency as the electromechanical resonant frequency, regardless of environmental factors or differences between parts, so that it is possible to obtain the maximum efficiency and output at any time.
The inverter INV inverts the phases of drive signals to be applied to two piezoelectric plates 32 and 33 or 32 and 34 provided on both sides of the piezoelectric ultrasonic motor 30. This allows drive signals out of phase by 180 degrees to be applied to the two piezoelectric plates 32 and 33 or 32 and 34.
To enable the piezoelectric ultrasonic motor 30 to adjust the rotational direction, the common piezoelectric plate 32 is provided on one surface of a metal body 31, and the two piezoelectric plates 33 and 34 are provided respectively on two divided sections of another surface of the metal body 31, which is opposite to the one surface. The common piezoelectric plate 32 is connected to one end of the inverter INV and the piezoelectric plates 33 and 34 are connected to the other end of the inverter INV through a switch SW.
If the switch SW is controlled to connect the output of the inverter INV to the upper piezoelectric plate 33, the common piezoelectric plate 32 and the upper piezoelectric plate 33 contract and expand to cause a bending deformation in the metal body 31, thereby rotating the metal body 31 clockwise. On the other hand, if the switch SW is controlled to connect the output of the inverter INV to the lower piezoelectric plate 34, the common piezoelectric plate 32 and the lower piezoelectric plate 34 contract and expand to cause a bending deformation in the metal body 31, thereby rotating the metal body 31 counterclockwise.
The piezoelectric ultrasonic motor drive circuit of FIG. 3 uses a two-phase driving scheme as with that of FIG. 2, and can prevent changes in the frequency through self-oscillation. In the conventional drive circuit configured as described above, it is necessary to ground the metal body 31 of the piezoelectric ultrasonic motor 30. However, since the piezoelectric plates 32, 33, and 34 are attached to the metal body 31, it is difficult to form an electrode for grounding the metal body 31. In addition, since the structure of the piezoelectric ultrasonic motor has been altered to allow changes in the rotational direction of the motor, it is difficult to change the rotational direction of a piezoelectric ultrasonic motor having a structure different from that shown in FIG. 3.